Windows XP install query

I recently installed WInXP on my machine. In total it came to about 1.8gig. Then I installed SP2 over the internet (a patch of about 110meg I believe) and it bumped it up to 4+Gig on my c: drive. This isnt much of a problem cos I have 10gig available but it is just confusing me as to where this space has gone. If you check the properties on C: it says 4gig but if you high light all on C: and check that it only comes to about 2gig. Where is this hidden data? Any one else encountered this problem?